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The Turtlebot handles this in the Turtlebot Node, which forms the ROS interface to the iRobot Create hardware.

The most relevant bits are:

Line 398 and forward, where if there is a cmd_vel to be sent to the hardware, it first executes the self.check_bumpers function at Line 429.

def check_bumpers(self, s, cmd_vel):
    # Safety: disallow forward motion if bumpers or wheeldrops
    # are activated.
    # TODO: check bumps_wheeldrops flags more thoroughly, and disable
    # all motion (not just forward motion) when wheeldrops are activated
    forward = (cmd_vel[0] + cmd_vel[1]) > 0
    if self.stop_motors_on_bump and s.bumps_wheeldrops > 0 and forward:
        return (0,0)
    else:
        return cmd_vel

The actual hardware interaction is taking place in the roomba_sensor_handler.py file or the create_sensor_handler.py file, also in the turtlebot_node package.